The CSEM survey technique is an important geophysical tool for prospecting for hydrocarbons in the earth's subsurface. In a CSEM survey, an electromagnetic-wave source (transmitter) generates an electromagnetic wave. The electromagnetic signal induced in the earth by the transmitter is recorded continuously in time by one or more receivers. The electromagnetic signal at a receiver location depends on physical properties, especially the electrical properties, of the medium through which the electromagnetic wave has passed from the source to the receiver. The behavior of this signal as a function of frequency and transmitter location or separation (offset) between transmitter and receiver can be used to estimate the spatially varying resistivity model of the subsurface. This estimated resistivity model is used for identifying hydrocarbons in the earth's subsurface.
In a typical marine CSEM survey, a constantly active electromagnetic-wave transmitter is towed along a line 11 above electromagnetic receivers 12 deployed on the seafloor 13, as illustrated in FIG. 1. For more details see for example Chapter 12, page 931 in Investigations In Geophysics No. 3, Electromagnetic Methods In Applied Geophysics, volume 2, edited by Misac N. Nabighian, Society of Exploration Geophysicists (1991). The receivers typically have multiple sensors designed to record different vector components of the electric and/or magnetic fields. The directions of the received data components by the receivers are indicated in FIG. 1 by arrows. Transmitter locations are illustrated by arrows along line 11 above the receivers. Normally, the separation between neighboring transmitter locations is much smaller than that between neighboring receivers. Transmitter-receiver offset is defined as the distance 14 between a transmitter location and a receiver location. The transmitting and receiving systems typically operate independently (without any connection between them), so that receiver data must be synchronized with shipboard measurements of the instantaneous transmitter position and electric current in the transmitter antenna by comparing clock times on the receivers to time from a shipboard or GPS (Global Positioning System) standard.
Every receiver records the electromagnetic signal continuously in time during a survey. The data recorded by one sensor (channel) on a receiver are called a common-receiver gather. This gather represents the electromagnetic signal at the receiver location induced by the source at all different source locations, or at different times during the survey. FIGS. 2A-C depict the measured electrical signals recorded by the three sensors of a receiver within a short time window (only a few transmitter waveforms) taken out of a common-receiver gather together with (FIG. 2D) the transmitter waveform that gave rise to it. FIG. 2A shows the measured x-component of the electric field, FIG. 2B the measured y-component and FIG. 2C the measured z-component. FIG. 2D shows the transmitter current signal of a square waveform with designed half period 4 s. An anomalous transmitter pulse 21 of 6 s width shows up on all three channels of the receiver. In practice, the transmitter signal may be a more complex waveform than the square wave depicted in FIG. 2D.
Marine CSEM data are typically interpreted in the temporal frequency domain. The resulting transformed data will have components at certain frequencies, determined by the frequency spectrum of the particular source waveform. After taking out the frequency-dependent effects of the source and the receiver itself, the signal at a frequency represents the response of the earth to an electromagnetic signal at that temporal frequency. Like any other type of wave, the electromagnetic signal in a CSEM survey has two attributes, amplitude and phase. The signals are therefore conveniently represented as complex numbers in either rectangular (real-imaginary) or polar (amplitude-phase) form.
In practice, the receiver data are usually converted to temporal frequency by dividing (or “binning”) the recorded time-domain data 31 into time intervals, e.g., bins x1, x2, and x3 in FIG. 3A, and determining the spectrum within each bin by standard methods based on the Fourier Transform. FIG. 3B shows the amplitudes of the spectral components from the bin x3. A typical bin length can be one or several periods of the transmitter waveform 32. Some methods of transforming data to the time-frequency domain include the Short-Time Fourier Transform (J. Allen, L. Rabiner, “A Unified Approach to Short-Time Fourier Analysis and Synthesis,” Proc. of the IEEE 65, 1558-64, (1977)); the Wavelet Transform (W. C. Lang and K. Forinash, “Time-frequency analysis with the continous wavelet transform,” Am. J. Phys. 66, 794-797, (1998)); the Wigner-Ville transform (E. Wigner, “On the quantum correction for thermodynamic equilibrium,” Phys. Rev. 40, 749-759, (1932)); the Choi-Williams transform (H. Choi and W. Williams, “Improved time-frequency representation of multicomponent signals using exponential kernels,” IEEE Trans. on Acoust., Speech, and Signal Processing 37, 862-871, (1989)); and the Bessel method (Z. Guo, L. G. Durand, and H. C. Lee, “The time-frequency distributions of nonstationary signals based on a Bessel kernel,” IEEE Trans. on Signal Proc. 42, 1700-1707, (1994)). The present invention is not limited to any particular method or methods for spectral decomposition of CSEM data to the temporal-frequency domain. In the temporal-frequency domain, signals, including both amplitude and phase, of each of the temporal-frequency components are functions of a bin designation variable (label).
With each temporal bin is associated a time, typically the Julian date at the center of the bin. Since the transmitter location is known as a function of time, these bins may be interchangeably labeled in several different ways: by Julian date of the bin center; by transmitter position; by the signed offset distance between source and receiver; by the cumulative distance traveled by the transmitter relative to some arbitrarily chosen starting point; or sometimes simply by a sequential bin number.
In many examples of CSEM hardware, data cannot be effectively recorded at the nearest offsets because the dynamic range of the receiver's digitizers is too small to accommodate the large dynamic range of the data. This region is sometimes known as the “saturation zone” and typically encompasses source-receiver offsets of less than about 500 meters. At far offsets, the controlled-source signal is too weak to be observed above the earth's magnetotellurics energy and other noises such as those induced by oceanic currents. This signal level may be called the “noise floor”.
Both amplitudes and phased of the CSEM data depend on the subsurface conductivities. Both of them need to be determined accurately in order to distinguish characteristics that signal the presence or absence of conductivity anomalies in the subsurface. Reliable phases of CSEM data are essential in applying inversion techniques for conductivity-anomaly detections. However, in a CSEM survey, the phase variations of CSEM data can be caused by many factors that are not related to the variations of the subsurface conductivity in the target zones. Some of these factors are identified in the following list:
(a) the transmitter current waveform deviates from the desired waveform, i.e., transmitter instability;
(b) the transmitter monitoring system fails to measure and/or report the actual transmitter current waveform accurately;
(c) transmitter and receiver signals are recorded separately using different time bases (clocks) that are not synchronized against a common GPS time base;
(d) the frequency-dependent responses of a receiver, such as receiver amplifiers, receiver antennae, and their combination, are not calibrated accurately;
(e) localized changes in the earth's resistivity close to a receiver mask the electromagnetic signals from the desired target zones.
Phase errors of the types of categories (a) and (b) above are common, and can occur in both land and marine CSEM surveys. As an example of the type of problem of category (c) above, if the time origin of the transmitter waveform (sometimes known as the “initial transmitter phase”) is in error by Δt, the phase of the receiver data in frequency domain at angular frequency ω will be incremented by an amount ωΔt. Similar factors arise on land, although it is easier to connect both the source and receivers to a common time reference, thus reducing the phase error associated with the clock synchronization.
Some techniques have been disclosed in the published literature for reducing the phase errors caused by some of the factors listed above. Direct measurement of the receiver-clock drift (time error) relative to a time reference (such as GPS) at the start and end of the survey allows users to stretch or compress measured data to an estimate of the reference time (Constable et al., “Marine magnetotellurics for petroleum exploration Part 1: A sea-floor equipment system,” Geophysics 63, 816-825 (1998)). This correction can reduce the error caused by the receiver clock drifts. Laboratory measurements of the response of the receiver's amplifier-antenna system have been used to compensate field CSEM data (Ellingsrud, et al., “Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola,” The Leading Edge 21, 972-982 (2002)).
As pointed out above, if the time origin of the transmitter waveform is in error by Δt, the phase of the receiver data in frequency domain at angular frequency ω will be incremented by an amount ωΔt. This type of error can also be caused by undesired variation of the transmitter waveform. To resolve the problems of transmitter instability and timing error associated with the transmitter (initial phase error), the transmitter current is usually recorded continuously during the survey by the source monitoring system. An independent monitoring receiver has also been mounted to the towed underwater transmitter previously to monitor the transmitter current that is actually injected into the water (MacGregor et al., “The RAMESSES experiment-III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57° 45′ N,” Geophys. J. Int. 135, 773-789 (1998)). In all cases, the transmitter signals recorded by the monitoring system in the time domain are used to identify and compensate the phase shifts caused by anomalous transmitter signal variations during the survey.
In principle, a source deconvolution technique can be used to obtain the response of the subsurface at each frequency once the source signal is known reliably even though the source may exhibit undesired variations (Ö. Yilmaz, Seismic Data Processing, Vol. 2 in Investigations in Geophysics, E. B. Neitzel, ed., Society of Geophysics, 498-506 (1987)). However, the deconvolution technique strongly depends on the fidelity of the recorded source signals during the survey. This dependence combined with the shortcomings of the deconvolution technique itself makes the application of this technique to CSEM data difficult. Instead, in CSEM data processing, one continuous transmitter towline is normally split into multiple sublines. Time windows of erratic variation in transmitter signal are skipped. Each subline uses a different transmitter initial time (or initial phase), determined from the transmitter current signals, for processing. The major shortcoming of this method and the deconvolution technique is the dependence on the measured transmitter signals. The measured transmitter signals may not be reliable and/or available due to malfunctions or failures of the source monitoring system. Also, splitting one towline into multiple sublines renders CSEM data processing tedious and error-prone. After data processing with the transmitter initial times (or phases), normally the data still have residual phase errors that need to be corrected by other methods.
Another method to detect and correct phase errors is to determine any timing error for one common-receiver gather by utilizing the frequency scaling behavior of the electromagnetic field in a uniform medium (PCT International Patent Application No. PCT/US06/46329). The phase of the electromagnetic field from a dipole in a uniform medium is a function of (R√{square root over (ω)}), where R is the distance between the transmitter and the receiver (called offset), and ω is the angular frequency. In a homogeneous-earth model, the phase variations with the frequency-scaled transmitter-receiver offset, (R√{square root over (ω)}), are the same for different frequencies. In a one-dimensional layered-earth model, data at small transmitter-receiver offsets shows a similar scaling behavior. Separation of the phase versus scaled-offset curves at different frequencies within a small offset range (e.g. within 2 km but outside the saturation zone) indicates a time error assuming the conductivity variation within the small offset range is small. A timing error can be determined by maximizing the overlap of the phase versus scaled-offset curves of different frequencies at near offset. The timing error determined from this method includes components from both transmitter and receiver clocks. This method is effective in determining the global phase shift for a common-receiver gather, ensuring that the phase goes to zero as the source and receiver offset goes to zero, as required by the laws of physics. However, this method is not valid when near-offset data (such as those with offset less than 1 km) do not exist or the undesired transmitter variations happen at locations too far from the receiver location for the frequency-scaling behavior of the phase to be valid.
What is needed is a method, applicable to both onshore and offshore CSEM surveys, that can detect, determine, and correct phase errors related to the source (i.e., categories (a) and (b) in the list above) based on data from normal, ordinary survey receivers, i.e. without using a source signal measured by a source monitoring system, so that data phase errors can be corrected when the source monitoring data are either not reliable or not available. The present invention satisfies this need.